1. Technical Field
The embodiments herein generally relate to communication systems, and more particularly to the field of orthogonal frequency division multiplexing (OFDM) communication systems.
2. Description of the Related Art
In conventional OFDM systems, timing and frequency offsets can cause large bit error rate and performance degradation. Timing offset refers to a delay in the received signal relative to the expected signal from the transmitter. Frequency offset refers to the difference between the frequency of a source and a reference frequency.
In a pilot based OFDM system, a traditional method for fine timing and frequency offset estimation is to detect the sub-carrier phase rotation on pilot bins across symbols. The mean phase rotation would indicate the fine frequency offset while the slope of phase rotation across the pilots would indicate the fine timing offset. In ISDB-T and ISDB-TSB, the pilots (referred to as scatter pilots) are placed four OFDM symbols apart in the time domain. For a non-static or a noisy channel, the channel may change substantially after four symbols. As the scatter pilots include a portion of channel changes, the phase rotation of the scatter pilots is generally not a good indicator of the frequency and timing offsets.
In an OFDM system, the phase rotation caused by the timing and frequency offset can be approximated as:Δφk≈ΔfTu+ξk  (1)as described in M. Speth et. al, “Optimum receiver design for wireless broad-band systems using OFDM—Part I”, IEEE Trans. on Communications, vol. 47, no. 11, pp. 1668-1677, November 1999, the complete disclosure of which, in its entirety, is herein incorporated by reference, where Tu is the OFDM symbol time, Δf is the frequency offset, ξ=ΔT/T is the timing offset in percentage, and k is the sub-carrier (bin) index. As illustrated in FIG. 1, the mean of the phase rotation is equal to ΔfTu and the slope of the phase rotation curve is the timing offset ξ.
The phase rotation in equation (1) can be obtained by taking the phase difference between two received data on the same bin between two adjacent symbols if the transmitted data on that bin is the same for the two symbols. In ISDB-T and ISDB-TSB, there are scatter pilots. The transmitted data on scatter pilots is always the same. In ISDB-T and ISDB-TSB, the phase difference can be taken between two received scatter pilots spaced four symbols away, and its relationship to the timing and frequency offset can be approximated as:Δφk≈4(ΔfTu+ξk)  (2)In the approximation shown in equation (2), a basic assumption is that the channel should stay almost unchanged for four symbols. In a high Doppler channel, the channel can change substantially, and the relationship in equation (2) may not hold true. As seen, with the increase in the rate of changes in the channel, the estimations of the timing and frequency offsets deteriorate considerably resulting in large bit error rates and poor system performance.